Electronic pipettes are used in the laboratory for metering fluids. They are known in different embodiments. Air cushion pipettes have an integral cylinder with a piston arranged therein. The cylinder is attached to an aperture in a fastening shoulder via a channel. A pipette tip can be releasably connected to the fastening shoulder. By displacing the piston in the cylinder, test fluid is drawn into the pipette tip or ejected therefrom. In this connection, the piston and cylinder do not come into contact with the fluid, as the piston moves the fluid indirectly via the air cushion. Only the pipette tip, which generally consists of plastics, is contaminated and can be exchanged after use.
Direct-displacement pipettes can be releasably connected to a syringe, of which the piston can be driven by means of the pipette in order to draw test fluid directly into the syringe and eject it therefrom. As the syringe is contaminated with the test fluid, it can be exchanged. The syringe also generally consists of plastics.
Pistonless pipettes can comprise a metering tip with a balloon-like end portion which is expanded to draw in test fluid and compressed to eject it. Such metering tips have also already been designed as exchangeable parts made from plastics.
When pipetting, the pipette dispenses the fluid received by the tip or syringe in one step. When dispensing, the fluid received by the syringe or the tip is dispensed in small quantities.
Multi-channel pipettes comprise a plurality of channels by means of which metering can take place simultaneously. Pipettes can be designed as hand-held apparatus and/or stationary apparatus.
All the aforementioned pipettes are electronic pipettes in the sense of this application. For precise metering of a volume of fluid, it is necessary to displace the piston in the cylinder or the displaceable element of a further displacement device, as precisely as possible, depending on the volume of fluid.
An electronic pipette is known from WO 91/16974 A1 which comprises a measurement device for measuring the distance travelled by the piston and a braking device controlled by a control device to arrest the piston. The braking device comprises grooves on the circumference of a rotating disc coupled to the electric drive motor and a cam which can be forced into a groove by a drive. With this pipette the piston is arrested by means of the brake as soon as the measuring device establishes that the piston has travelled the required distance for the desired metering. The mechanical braking of the piston by means of the brake is prone to wear and tear. As a result, trouble-free operation of the pipette over a lengthy period of use is not guaranteed.
Moreover, electronic pipettes are known in which the electric drive motor is coupled to a magnetic rotational angle sensor which comprises alternate magnetic poles on the circumference of a magnetic disc. A magnetic sensor is aligned with the circumference of the magnetic disc. The number of different magnetic poles on the circumference of the magnetic disc is restricted. Furthermore, the passing through of different poles can only be determined sufficiently reliably by means of the magnetic sensor. However, the exact position of the magnetic disc cannot be measured by the sensor in the space between the poles. As a result, the resolution of the magnetic rotational angle sensor and thus the precision of the metering is reduced. Moreover, oscillation controllers are possible with the rotational movements of the magnetic disc, which are limited by the spacing between the different poles.
Proceeding from this, the object of the invention is to provide an electronic pipette with trouble-free and precise control of the volume of fluid to be metered over a lengthy period of use.
U.S. Pat. No. 5,892,161 and WO 98/10265 A1 disclose a mechanical pipette with electronic display, which comprises a transducer arrangement to monitor the rotational movement of a volume delivery adjustment device of the pipette. The transducer arrangement preferably comprises two Hall-effect sensors which are spaced 90 rotational degrees apart from one another and detect the magnetic field of an annular magnet which is attached to the volume delivery adjustment device. The Hall-effect sensors produce sinusoidal signals which are 90° out of phase from one another. The signals are processed in order to determine the absolute position of the volume adjustment mechanism and to display the volume delivery adjustment of the pipette. The transducer arrangement, in conjunction with the electronics assembly, monitors both the number of revolutions of the volume adjustment mechanism from an initial position and the position of the volume adjustment mechanism within a revolution.
The electronic processing of the signals emitted by the Hall-effect sensors to establish precisely the rotational position of the magnetic sender disc is costly.